(i) Field of the Invention
The present invention relates to a high-performance nonaqueous electrolyte secondary battery using an organic solvent as an electrolyte solution, and more particularly, it relates to a negative electrode material for a lithium ion secondary battery.
(ii) Description of the Related Art
In recent years, as a power supply of a portable electronic device, demands for a secondary battery having a high energy density and excellent charge and discharge cycle properties are increasing. In this regard, the nonaqueous electrolyte secondary battery, especially a lithium ion secondary battery is very expected as a battery having a high voltage and a high energy density.
In particular, much attention has been nowadays paid to a battery system using a lithium-containing transition metal oxide as a positive electrode active material and a carbonaceous material as a negative electrode. For both positive and negative electrodes of this battery, there is utilized a mechanism of lithium ion intercalation, deintercalation, lithium ion doping or de-doping with respect to each active material, and hence, no metal lithium dendrite is formed even if the charge and discharge cycles are repeated, in contrast to a battery using metal lithium. This battery is, therefore, expected to exert the excellent charge and discharge cycle properties and safety.
At present, a carbon material is widely used as a negative electrode material for such a nonaqueous electrolyte secondary battery. As a proposal of using the carbon material as the negative electrode material, each of Japanese Patent Applications Laid-Open Nos. 208079/1982, 102464/1983, 192266/1983, 143280/1984 and 54181/1985 discloses the employment of, e.g., graphite as the negative electrode material. However, the crystallite of graphite is in the extremely developed state, and so, in the nonaqueous electrolyte secondary battery using such a negative electrode, the decomposition of the electrolyte solution is apt to occur as a side reaction at the hexagonal mesh surface ends of graphite crystals during charge, so that there is a problem that a charge and discharge efficiency and the charge and discharge cycle properties are poor.
In order to solve such a problem, there has been proposed the use of a carbon material which has a low graphitization degree and whose crystallite is not in the extremely developed state. Specifically, it has been suggested that the graphitization degree can be controlled by a calcination temperature, and a method using, as the negative electrode, a calcined organic material obtained at a calcination temperature of not more than 1500xc2x0 C. is disclosed in Japanese Patent Applications Laid-Open Nos. 93176/1993 and 235372/1985. In such a carbon material having the low graphitization degree, the decomposition of the electrolyte solution during charge can be more suppressed as compared with a carbon material calcined at a temperature not less than 2800xc2x0 C. and having the high graphitization degree.
The carbon material having the low graphitization degree has, however, lower charge and discharge efficiency than the material having the high graphitization degree, and its true density is also low. Therefore, the obtained battery energy density becomes low, which is insufficient as a battery capacity.
Accordingly, each of Japanese Patent Applications Laid-Open Nos. 059703/1998, 343196/1996, 368778/1992 and 66404/1992 discloses an attempt to suppress the side reaction such as decomposition of the electrolyte solution to thereby improve the battery properties by coating the surface of the carbon material with amorphous carbon or carbonaceous decomposition components to reduce the surface area of the carbon material or covering the hexagonal mesh surface end of the active graphite crystal.
Further, Japanese Patent Application Laid-Open No. 2428905/1993 discloses an attempt to suppress the side reaction such as decomposition of the electrolyte solution to thereby improve the battery properties by restricting a ratio of particles which have a small particle diameter to reduce the surface area of the graphite material.
When using the graphite material to a negative electrode of the nonaqueous electrolyte secondary battery, since a main cause of the irreversible capacity of the battery is the decomposition reaction of the electrolyte solution which occurs at the edge surface of the graphite crystal during charge, reducing the surface area of the negative electrode material or covering the material surface with the electrolyte solution and the inactive coating film are effective for improving the charge and discharge efficiency.
In general, as a technique for working the carbon particles into an electrode, there is widely used a method which comprises mixing the carbon particles with a binder, dispersing the resultant mixture in an aqueous or organic solvent to obtain a slurry, applying this slurry on a current collector, and then drying the same. The negative electrode manufactured by this method has, however, a high void ratio and small filling density as it is, and hence the energy density of the battery can not be sufficiently increased.
In particular, since the material from which the carbon particles having a small particle diameter are removed for reducing the specific surface area has inferior filling properties, it is hard to increase the electrode density. Therefore, the electrode manufactured by the above-described method is usually compressed by using a roll press machine or a uniaxial press machine in order to increase the filling density.
In case of manufacturing the electrode by using the conventional carbon material, however, when the electrode is tried to be compressed in the manufacturing process, the improvement effect is decreased and the charge and discharge halo efficiency becomes lower. Thus, the battery in which such a carbon material is used for the electrode can not be compressed to sufficiently increase the filling density of the electrode, and the energy density of the battery becomes lower, which leads to the insufficient battery capacity.
The present inventor has discovered that the reason for the fact that the charge and discharge efficiency lowers by pressure compression of the electrode is due to the increase in the specific surface area of the electrode. Since compressing the electrode decreases the void ratio of the electrode, the surface area of the electrode is seemingly reduced. However, when the electrode is compressed to, the specific surface area of the electrode is surprisingly greatly increased.
That is because compressing the electrode causes the carbon particles to be fragmentized. When the carbon particles are fragmentized, the specific surface area of the electrode is increased, which readily provokes decomposition reaction of the electrolyte solution, thereby lowering the charge and discharge efficiency. Further, in case of the graphite material having the surface covered with amorphous carbon, when the carbon particles are fragmentized, the active carbon hexagonal mesh surface edge which is not covered with amorphous carbon is exposed, and the effect obtained by coating is thus reduced. Therefore, deterioration of the charge and discharge efficiency due to compression becomes prominent.
Moreover, Japanese Patent application Laid-Open No. 214615/1998 discloses that before the adhesion of amorphous carbon onto the surfaces of the graphite particles, the graphite particles are treated with potassium permanganate, which permits amorphous carbon to further firmly adhere thereto. It was, however, difficult to increase the filling density without deteriorating the charge and discharge effect by this method.
Japanese Patent Application Laid-Open No. 27316/1997 discloses that the graphite-based carbon and amorphous carbon are mixed to be used as a negative electrode active material. Additionally, Japanese Patent Application Laid-Open No. 153514/1996 discloses that a multilayer film having a graphite layer and an amorphous carbon layer or a film formed of a mixture containing graphite and amorphous carbon is used as a negative electrode. Although these examples achieve their own objects, they are different from an object of the present invention, i.e., efficiently bringing out the properties of the graphite material whose surface is covered with amorphous carbon which causes no side reaction such as decomposition of the electrolyte solution.
In view of the above-described problems, it is an object of the present invention to provide a high-performance nonaqueous electrolyte secondary battery having the excellent charge and discharge efficiency.
The present invention is directed to a rechargeable nonaqueous electrolyte secondary battery comprising a positive electrode which can be doped with lithium ions and de-doped of lithium ions, a nonaqueous electrolyte solution and a negative electrode, wherein a negative electrode active material consists essentially of a carbon material (which may be referred to as a xe2x80x9cnegative electrode carbon materialxe2x80x9d for clarification hereinafter) including at least two component: (a) flake graphite particles; and (b) a non-flake graphite material whose surface is covered with amorphous carbon.
Moreover, the present invention is also directed to a method for manufacturing a nonaqueous electrolyte secondary battery, said method comprising steps of: applying a slurry onto a current collector; the slurry comprising (a) flake graphite particles, (b) a non-flake graphite material whose surface is covered with amorphous carbon, a binder, and a dispersion medium; drying the slurry; and compressing the dried slurry by the application of a pressure.
Furthermore, the present invention is directed to a carbon material composition comprising: (a) flake graphite particles; and (b) a non-flake graphite material whose surface is covered with amorphous carbon; a weight ratio of (a) to (b) being 10:90 to 70:30.
According to examination by the present inventor, as described above, in case of manufacturing a negative electrode by using the covered graphite material as a negative electrode active material, when the negative electrode active material is compressed in order to increase the filling density, the active carbon hexagonal mesh surface edge which is not covered with amorphous carbon is exposed, deterioration of the charge and discharge efficiency due to compression is extremely large.
On the other hand, the flake graphite particles have a small friction coefficient, which is preferable for increasing the filling density. However, when only the flake graphite particles are used as the negative electrode active material, the graphite particles are uniformly oriented by pressure compression. Which leads to degradation of the wettability of the electrode and the electrolyte solution. Therefore, the negative electrode hardly impregnates with the electrolyte solution, and the coefficient of effective utilization is hence decreased, thereby reducing the capacity of the battery.
As a countermeasure, the present invention uses both the non-flake graphite material those surface is covered with amorphous carbon and the flake graphite particles to be mixed. As a result as compared with the case where the graphite material whose surface is covered with amorphous carbon is solely used, the filling density can be increased with a smaller pressure, and it is possible to suppress deterioration of the battery properties due to fracture of the graphite particles whose surfaces are covered with amorphous carbon. Since the flake graphite particles have a small friction coefficient (it can be considered that they function as cushions because they are squashy), the friction resistance between the graphite material particles whose surfaces are covered with amorphous carbon becomes small, and whereby the particles are apt to be shifted from each other during compression, increasing the filling properties. Therefore, the negative electrode according to the present invention can sufficiently increase the density with a small pressure, and fracture of the graphite particles whose surfaces are covered with amorphous carbon can be suppressed.
Since it is possible to absorb the uniform orientation of the flake graphite particles during pressure compression, appropriate irregularities can be formed on the surface of the negative electrode. Therefore, the negative electrode according to the present invention is superior in the wettability to the electrolyte solution, as compared with the negative electrode using only the flake graphite particles as the negative electrode active material.
Therefore, according to the~present invention, it is possible to obtain a high-performance nonaqueous electrolyte secondary battery which exerts a high charge and discharge efficiency.